The present state of the art in making "silver-sand" type current-limiting fuses is to use a silver fuse element surrounded by silicon dioxide sand, as a filler. This type of silver-sand fuse has been very effective in "limiting", or forcing the current to a very low value as soon as the fuse element is melted, or vaporized. This can be seen in oscillograms of typical fuse operations. However, for certain sizes, ratings, currents, etc., current can continue to flow through the fuse, due to the high conductivity of the sand filler after the initial current-limiting action. This can be seen as a "secondary" current-flow on oscillograms of such fuse operations.
Reference may be had to U.S. Pat. No. 3,213,242, issued Oct. 19, 1965 to Frank L. Cameron, and assigned to the assignee of the present invention, for a description of the current-limiting action, which takes place utilizing sand as a filler material. In addition to sand, this U.S. Pat. No. 3,213,242 utilizes a layer of calcium carbonate (CaCO.sub.3). The layer of CaCO.sub.3 is utilized between the layers of sand in the fuse to obtain specific melting characteristics, and a good low-current clearing ability.
It is, of course, desirable to limit the amount of energy released within the fuse during fuse rupture. Although this is measured by the square of the current, nevertheless the area under the current-time curves give a rough approximation of the energy released within the fuse tube. With fuses of the prior art, utilizing sand only as the filler granular material, I have observed a "secondary" current released very shortly after the main let-through current has ceased. It is, of course, desirable to eliminate the presence of such a "secondary" current, as described hereinbefore with the prior-art types of fuses as it adds to the energy released within the fuse.